code: def matvec(x):
      """Needed for test_pickle as local functions are not pickleable"""
      return np.zeros(3)
  
imports: from functools import partial
  from itertools import product
  import operator
  from pytest import raises, warns
  from numpy.testing import assert_, assert_equal
  import numpy
  import scipy.sparse
  import scipy.sparse.linalg._interface
  from scipy.sparse._sputils import matrix
label_desc: Use image node_id 4 for calling a global function with example usage: matvec(x) and returns: np